The purpose of this study is to characterize, in a rigorous fashion, the critical period marking the transition from health to illness (phenocoversion) in individuals who carry the Huntington's Disease (HD) gene, and to determine a methodology for the rational design of clinical trials of experimental therapeutic agents aimed at postponing clinical onset in presymptomatic gene carriers. The prospective determination of the phenoconversion rate is necessary for sample size estimation for such trials. In addition, a careful consideration of feasibility and ethical issue (such as informed consent and confidentiality) in the setting of the pilot observational study is essential before testing any therapeutic intervention in this population.